There's also some need to avoid assuming that democrats don't have bigoted ideas, at all. And if you combine everything, and look at the timing, there's a pretty clear "Trump effect", in which an increasing amount of lunacy drives people away. In the image, note that the first 7 is 2018.
It does beg the question, then, if we had a real moderate left party and a real moderate conservative party, would it stay that way? I'm inclined to imagine it probably wouldn't. So it's largely down to the abject extremism in the GOP. That creates potential to shift back if they calmed down.